Validating for specific day is not working ?

[antdp97]

I'm trying to validate cron pattern like this:
0 0 * * MON
but not sure why the result of isValidCron function is false. I have to add a space next to second 0 to get the true result. 0 0 * * MON
The same happens for other days like "TUE WED ... " but not for this case
0 8 * * *